Moderate regulated deficit irrigation can increase olive oil yields and decrease tree growth in super high density ‘Arbequina’ olive orchards

• RDI produced a compact tree that facilitated mechanical harvest and fruit removal.
• The use of SWP as a tool in irrigation scheduling for olives is promising.
• Moderate RDI water stress (−2.2 MPa) significantly increased oil yields.
• Moderate RDI water stress increased oil polyphenol content.

A number of studies have found that ‘Arbequina’ olive oil yields in super high density (SHD) orchards are maximized at water application rates below 100% of full irrigation. To evaluate this relationship further, four regulated deficit irrigation (RDI) treatments ranging from 58 to 30% ETc were initiated about 70 days after full bloom and evaluated over 4 years. The experiment was located in two different soil types in the same orchard, a loam and a gravelly loam soil which significantly influenced water stress. Few significant differences were found between the different irrigation treatments; however, gravelly loam soils imposed significantly lower stem water potentials (SWP) than found in the loam soil. The loam and gravelly loam soils imposed a mild (ψstem = −1.6 MPa) and moderate tree (ψstem = −2.1 MPa) water stress, respectively, during the oil accumulation period. Oil yields were significantly greater and fruit yields were significantly lower in moderately water stressed trees versus the mildly stressed trees in 3 of the 4 years of the study. Moreover, oil percentages were significantly greater in moderately stressed trees months before tree harvest. A number of factors likely contributed to this increase in oil yield including: (1) improving the light environment for oil accumulation, (2) hastening fruit maturity at harvest, and (3) increasing fruit removal percentages from the trees. Tree growth was significantly decreased in the moderately stressed trees. Decreases averaged 36%, 67%, and 46% for shoot growth, trunk growth, and pruning weights, respectively, over the 4 years of the study. This resulted in a smaller, more compact tree in the moderate water stressed trees which facilitated mechanical harvest in SHD orchards.